1. Field of the Invention
The present invention relates generally to integrated circuit manufacture; and more particularly to a non-invasive, in-situ method and apparatus for detecting end-point of etch processes.
2. Description of the Related Art
A pervasive trend in modern integrated circuit manufacture is to produce transistors having feature sizes as small as possible. To produce a high-density integrated circuit efficiently, semiconductor processes include the production of complex circuits on a single monolithic substrate, thereby allowing relatively large circuit systems to be incorporated on a single and relatively small die area. Further, many such die are commonly produced on a single wafer which, after production, is diced into the plurality integrated circuits.
The benefits of high-density circuits can only be realized if advanced processing techniques are used. For example, semiconductor process engineers and researchers often study the benefits of electron beam lithography and x-ray lithography to achieve the higher resolutions needed for sub-micron features. To some extent, wet etch has given way to a more advanced anisotropic (dry etch) technique.
Plasma etching and related dry etch processes such as reactive ion etching are becoming increasingly important in the field of semiconductor device manufacture. In general, these processes involve the exposure of one or more wafers containing a number of semiconductor devices to a chemical atmosphere that has been ionized by the application of radio frequency energy. The usual goal of such processes is to remove exposed portions of an underlying layer while leaving an overlying layer. The overlying layer is typically a patterned photoresist and remaining portions of the underlying layer form features of the integrated circuit.
As the feature size of devices manufactured by these processes becomes smaller, it becomes increasingly necessary to accurately define the endpoint of the etching process. The end point is the point at which the desired portions of the underlying layer have been removed by the plasma introduced into the chamber holding the semiconductor wafer. One method of performing endpoint detection is generally referred to as laser endpoint detection and involves the illumination of a predetermined portion of the wafer with energy from a laser and the analysis of the reflected energy.
Laser endpoint detection is fundamentally an interferometric technique. Accordingly, it requires an optical window for monitoring the ongoing etch process. By way of example, in a parallel plate type of plasma reactor wherein a semiconductor wafer is placed upon a bottom plate and wherein a second plate is placed above the wafer to define a space for the induction of a plasma, an optical window is located to be aligned with the defined space. An optical measuring device, by way of example, a chromator, is placed adjacent to the window for measuring a specified radio frequency generated by the plasma and the wafer during the etch process. The chromator is used to detect a specified frequency threshold of the reflected energy from the laser beam.
The specified frequency threshold is a specified frequency of a radio frequency light beam emitted during the dry plasma etch process. The actual frequency of the emitted light is a function of the exposed semiconductor material. The end point detection (the point at which a semiconductor circuit is adequately etched) occurs when exposed portions of an underlying layer on the wafer is completely etched away. End point for a batch of wafers occurs when exposed portions of the underlying layer being etched is completely etched away at its thickest point on the wafer. Thus, if etching is terminated prior to actual end point, the thickest portion of the exposed underlying layer being etched will remain on the wafer. If etching is terminated after end point, over-etching will occur and over-etching consequences will occur. For example, undercutting of "non-etch" regions may occur which often affects the speed distribution of the part being etched, even for undercutting of as little as fifty angstroms.
A problem with this process is that a misalignment of the chromator with respect to the window can have detrimental effects in that the etch process controls do not function properly. The reason is that a misaligned chromator can miss endpoint because it fails to detect the specified frequency from the reflected laser beam.
The consequences of a misaligned chromator can be severe. In some instances, a misaligned chromator can result in the endpoint of the etch process being detected up to a second late. Late detection of the endpoint thus results in an etch process being continued after it should have been terminated. The result is that the wafer is subjected to excessive etching. The excessive etching, at a minimum, reduces operational reliability. More significantly, the excessive etching can result in the semiconductor wafer being ruined or scrapped. As a typical semiconductor wafer can be worth $20,000 to $60,000, and a single lot of wafers can be worth $1,500,000, the economic consequence of a misaligned chromator is significant. Over the course of a year, the cumulative effect of occasionally misaligned chromators can be dire.
A misaligned chromator can also result in monitoring equipment merely misdiagnosing a wafer as being a defective wafer based upon measurement results. A good wafer may thus be discarded in such a situation once again resulting in huge economic waste. Thus, there exists a need in the art for an improved apparatus and method for monitoring the endpoint of an etch process that reduces the likely hood of misalignment.